The present invention relates to wireless power supply systems, and more particularly to systems and methods for communicating in a wireless power supply system.
Many conventional wireless power supply systems rely on inductive power transfer to convey electrical power without wires. A typical inductive power transfer system includes an inductive power supply that uses a primary coil (or a transmitter) to wirelessly transfer energy in the form of a varying electromagnetic field and a remote device that uses a secondary coil (or a receiver) to convert the energy in the electromagnetic field into electrical power. Recognizing the potential benefits, some developers have focused on producing wireless power supply systems with adaptive control systems. Adaptive control systems may give the wireless power supply the ability to adapt operating parameters over time to maximize efficiency and/or control the amount of power being transferred to the remote device.
Conventional adaptive control systems may vary operating parameters, such as resonant frequency, operating frequency, rail voltage or duty cycle, to supply the appropriate amount of power and to adjust various operating conditions. For example, it may be desirable to vary the operating parameters of the wireless power supply based on the number of electronic device(s), the general power requirements of the electronic device(s) and the instantaneous power needs of the electronic device(s). As another example, the distance, location and orientation of the electronic device(s) with respect to the primary coil may affect the efficiency of the power transfer, and variations in operating parameters may be used to optimize operation. In a further example, the presence of parasitic metal in range of the wireless power supply may affect performance or present other undesirable issues. The adaptive control system may respond to the presence of parasitic metal by adjusting operating parameters or shutting down the power supply. In addition to these examples, those skilled in the field will recognize additional benefits from the use of an adaptive control system.
To provide improved efficiency and other benefits, it is not uncommon for conventional wireless power supply systems to incorporate a communication system that allows the remote device to communicate with the power supply. In some cases, the communication system allows one-way communication from the remote device to the power supply. In other cases, the system provides bi-directional communications that allow communication to flow in both directions. For example, the remote device may communicate its general power requirements prior to initiation of wireless power transfer and/or real-time information during wireless power transfer. The initial transfer of general power requirements may allow the wireless power supply to set its initial operating parameters. The transfer of information during wireless power transfer may allow the wireless power supply to adjust its operating parameters during operation. For example, the remote device may send communications during operation that include information representative of the amount of power the remote device is receiving from the wireless power supply. This information may allow the wireless power supply to adjust its operating parameters to supply the appropriate amount of power at optimum efficiency. These and other benefits may result from the existence of a communication channel from the remote device to the wireless power supply.
An efficient and effective method for providing communication in a wireless power supply that transfers power using an inductive field is to overlay the communications on the inductive field. This allows communication without the addition of a separate wireless communication link. One common method for embedding communications in the inductive field is referred to as “backscatter modulation.” Backscatter modulation relies on the principle that the impedance of the remote device is conveyed back to the power supply through reflected impedance. With backscatter modulation, the impedance of the remote device may be selectively varied to create a data stream (e.g., a bit stream) that is conveyed to the power supply by reflected impedance. For example, the impedance may be modulated by selectively applying a load resistor to the secondary circuit. The power supply monitors a characteristic of the power in the tank circuit that is impacted by the reflected impedance. For example, the power supply may monitor the current in the tank circuit for fluctuations that represent a data stream.
As the industry grows to use wireless power supply systems that supply power wirelessly to multiple remote devices, communication between the power supply and multiple remote devices has become more important. Devices may have specific power needs that affect performance of the wireless power supply, and may desire to communication those needs. However, with multiple remote devices attempting to communicate, there is a possibility of data collision and data loss. Loss of communication may prevent the devices from communicating power requirements and possibly result in too much or too little power being transferred, potentially damaging a device. There also may be too much or too little power transferred if one or more of the remote devices is incapable of communicating information to the wireless power supply.
Conventional wireless power transfer systems have employed separate communication channels, such as Bluetooth or other RF communication systems, to manage communication with multiple devices. These systems, however, add cost and complexity to the remote devices and the wireless power supply. And, the long distance capabilities of RF communication systems may result in communication with remote devices not present near the wireless power supply. For example, if two wireless power supplies, each associated with a remote device to be charged, are in the same room and use an RF communication system, there may be communication errors with the remote devices during power transfer.